The present invention relates to a method and apparatus for the cleansing of environments containing compounds which contain nitrogen and oxygen (including nitrogen oxide waste environments) and the conversion of nitrogen oxides to fertilizer.
Many types of processes and industrial environments generate compounds containing nitrogen and oxygen, including nitrogen oxides, as effluents or waste materials. Any process which provides a sufficiently hot metal surface in contact with air can cause the formation of nitrogen oxides (usually as NO, NO.sub.2, or N.sub.2 O.sub.2), with the heated metal surface acting as a catalyst. Metal finishing processes, certain etching processes, and chemical syntheses can produce nitrogen oxides as a by-product. Compounds containing nitrogen and oxygen are also used in commerce or are produced as by-products in certain processes.
Although nitrogen oxide emissions have not received as widespread attention as sulfur oxide emissions, the nitrogen oxides similarly form acids when combined with water. Nitrous acid and nitric acid are relatively strong acids with high pKa values which may be harmful to the environment and hazardous to the health of persons or animals which come into contact with the oxide or the acid.
Hydrogen peroxide (H.sub.2 O.sub.2), with and without acids or bases, has been used to remove NOx and other acid gases from combustion flue gases, metal pickling operations, fluidized bed gas scrubbers, spray dryers, and nuclear fuel processing operations. That process with acids converts the nitrogen oxides to nitric acid, assuring that there is little nitrous acid remaining in the stream ("The Use of Hydrogen Peroxide for the Control of Air Pollution," Stud. Environ. Sci., 34, 275-292, (1988), CA110(18); 159688s; "Gas Scrubber Using and Alkali Solution Containing Hydrogen Peroxide," Japanese patent JP 48101378, CA80(26): 148789 m; "Absorbing Nitrogen Oxides from a Waste Gas with a Solution Containing Hydrogen Peroxide, Hydroxide and Cupric or Ferrous Ions," Japan Patent JP 49008465, CA80(26): 148804n; "Nitrogen Oxide Removal from Gases by Scrubbing." Japan Patent JP 52085979, CA88(10): 65480t; and "Conversion of Nitrogen Oxides to Potassium Nitrate in Waste Gas Treatment," Japan Patent JP 50033981, CA88(10): 65480t.). However, combustion flue gas scrubbing has been the primary use of hydrogen peroxide in scrubbers, and these streams usually contain numerous effluents, including sulfur oxides.
Several scrubber liquors have been used and/or proposed for the removal of NO.sub.x, for example, an aqueous suspension of magnesium carbonate and magnesium hydroxide.sup.(2) ; a solution of vanadium in nitric acid.sup.(3) ; ammonium sulfide and ammonium bisulfide.sup.(4) ; milk of lime.sup.(5) ; ammonium.sup.(6) ; urea.sup.(7) ; sodium sulfite and sodium hydroxide.sup.(8,9) ; sodium hydroxide.sup.(1,9) ; hydrogen peroxide.sup.(10,11) ; and sodium hydroxide and hydrogen peroxide.sup.(12,21,25).
Hydrogen peroxide with and without acids or bases has been used to remove NO.sub.x and other acid gases from combustion flue gases, metal pickling operations, fluidized bed gas scrubbers, spray dryers, and nuclear fuel processing operations. However, combustion flue gas scrubbing has been the primary use of hydrogen peroxide in scrubbers. Hydrogen peroxide alone has been added to a single column of a multiple column flue gas system.sup.(13,14) or to the entire scrubber solution.sup.(15-17) with an efficiency for NO.sub.x that was greater than 90 percent. Hydrogen peroxide has been blended with nitric and/or sulfric acids to improve the scrubber efficiency when added to a single column of a multiple column flue gas system, which showed significant improvement in the NO.sub.x removal.sup.(18,19). Sodium or potassium hydroxide with hydrogen peroxide has been used to improve scrubber efficiency for flue gases.sup.(21) and for general NO.sub.x removal from gas streams.sup.(22-25). For example, the efficiency for nitric oxide (NO) and nitrogen dioxide (NO.sub.2) removal improved from 3.8 to 46 percent, respectively, for 1-molar KOH to 91 and 98 percent, respectively, when 0.12-molar H.sub.2 O.sub.2 was added to the 1-molar KOH.sup.(22). Addition of 50-ppm Cu.sup.+2 (or Fe.sup.+2) improved the efficiency of a 5-percent NaOH/3 -percent H.sub.2 O.sub.2 solution from 80.6 to 93.5.sup.(23). Another use of hydrogen peroxide with sodium hydroxide to improve the removal of NO.sub.x involved an initial scrub with NaClO.sub.2, which produced ClO.sub.2 that was absorbed by NaOH and H.sub.2 O.sub.2 in a second column.sup.(25). For this example, the NaClO.sub.2 /NaOH/H.sub.2 O.sub.2 system had a removal efficiency of 98.6 for NO.sub.x. Addition of hydrogen peroxide to metal pickling baths has been used to lower the NO.sub.x emissions.sup.(26-28). In fact, hydrogen peroxide is used in several analytical methods to oxidize NO and/or the nitrite ion to improve the performance of impingers used for sampling NO.sub.x emissions.sup.(12,29-31). Hydrogen peroxide has been used in nitric acid plants to remove the tail gas which contains a mixture of NO and NO.sub.2. These previous studies illustrate that hydrogen peroxide has the potential to oxidize the NO and NO.sub.2.sup.-1 in the John F. Kennedy Space Center (KSC) scrubber liquor.
Hydrogen peroxide is reported to be unstable in acid or basic solutions with the maximum stability near a pH of 4.sup.(32-34). However, the largest single use of hydrogen peroxide in the United States is cotton bleaching, where most operations use stabilized alkaline hydrogen peroxide systems. Alkali and alkaline earth silicates (the most effective), phosphates, and organic chelating agents have been used as alkaline stabilizers for hydrogen peroxide.
REACTION OF NO.sub.2 WITH WATER. Nitric acid is a very important commercial product that is used as an intermediate in the manufacture of fertilizer. The production process uses the absorption of oxides of nitrogen into water and dilute acids. Because of this industrial need, the absorption process has been extensively examined for over a hundred years and in spite of this effort, the complex chemistry is not fully understood. There have been many theories proposed to describe transport from the gas phase to the final product. Reactions that relate to the oxidizer scrubber process, the absorption processes, and the effects of hydrogen peroxide have been summarized in the following sections.
Reaction Mechanisms. The mechanism for the absorption of the equilibrium mixture of nitrogen dioxide and nitrogen tetroxide, which is sometimes called "nitrogen peroxide" and designated as NO.sub.2 *, has been the subject of many investigations.sup.(35-48). There are four oxides of nitrogen that need to be considered when examining the reaction of nitrogen dioxide with a water-based scrubber liquor. These species [nitrogen dioxide, nitrogen tetroxide, nitric oxide, and dinitrogen trioxide (N.sub.2 O.sub.3)] exist in equilibrium with one another. Nitric oxide, although not an initial component of the reaction of nitrogen peroxide with water, is formed by the decomposition of nitrous acid in the liquid phase and released back to the gas phase. The reactions of nitrogen tetroxide, dinitrogen trioxide, and nitrogen dioxide in acidic and basic solutions are given below along with the associated electrochemical half-cell reactions.
______________________________________ Reactions of N.sub.2 O.sub.4 2NO.sub.2(g) N.sub.2 O.sub.4(g) (1-1) N.sub.2 O.sub.4(g) N.sub.2 O.sub.4(l) (1-2) N.sub.2 O.sub.4(l) + H.sub.2 O.sub.(l) HNO.sub.3(l) + HNO.sub.2(l) (1-3) 4HNO.sub.2(l) N.sub.2 O.sub.4(l) + H.sub.2 O.sub.(l) + 2NO.sub.(l) (1-4) N.sub.2 O.sub.4(l) NO.sup.+1 + NO.sub.3.sup.-1 (1-5) NO.sup.+1 + H.sub.2 O HNO.sub.2 + H.sup.+1 (1-6) Reactions of N.sub.2 O.sub.3 NO.sub.(g) + NO.sub.2(g) N.sub.2 O.sub.3(g) (1-7) N.sub.2 O.sub.3(g) N.sub.2 O.sub.3(l) (1-8) N.sub.2 O.sub.3(l) + H.sub.2 O.sub.(l) 2HNO.sub.2(l) (1-9) Oxidation of HNO.sub.2(l) 2HNO.sub.2(l) + O.sub.2(l) 2HNO.sub.3(l) (1-10) Direct Absorption of NO.sub.2(g) NO.sub.2(g) NO.sub.2(l) (1-11) 2NO.sub.2(l) + H.sub.2 O.sub.(l) HNO.sub.3(l) + HNO.sub.2(l) (1-12) Gas Phase Reactions of NO.sub.2(g) and N.sub.2 O.sub.4(g) 3NO.sub.2(g) + H.sub.2 O.sub.(g) 2HNO.sub.3(g) + NO.sub.(g) (1-13) N.sub.2 O.sub.4(g) + H.sub.2 O.sub.(g) HNO.sub.3(g) + HNO.sub.2(g) (1-14) 3HNO.sub.2(g) HNO.sub.3(g) + 2NO.sub.(g) + H.sub.2 O (1-15) HNO.sub.3(g) HNO.sub.3(l) (1-16) ______________________________________
Reactions 1-1 through 1-16 illustrate the major steps that are thought to occur during the absorption of nitrogen peroxide into water or water solutions. The primary reactions in the liquid and gas phases are the hydrolysis reactions of nitrogen tetroxide to form nitrous and nitric acids in equal molar quantities, see reactions 1-3 and 1-14. Nitrous acid, formed in these reactions, is a weak acid with an ionization constant of 4.5.times.10.sup.-4, it decomposes reversibly at a measurable rate to produce nitric oxide and nitric acid. The series of reactions 1-4 through 1-6 are associated with the liquid phase decomposition of nitrous acid and reaction 1-5 is considered to be the rate controlling step.
Reactions 1-7 through 1-10 describe the interaction of N.sub.2 O.sub.3 with water to form HNO.sub.2(1) as the only product. Although formation of N.sub.2 O.sub.3(g) is not highly favored, its reactions with water are very fast, which makes its contribution to the formation of HNO.sub.2(1) significant.
Formation of nitric oxide in the gas phase (see reactions 1-4, 1-13, and 1-15) from solutions with different initial pH values, has been used to indicate if a reaction occurred in the liquid or gas phase. Alkaline solutions were thought to prevent the formation of NO by preventing reaction 1-15; however, several investigators.sup.(39,41,42,44) have observed the formation of NO in strong sodium hydroxide solutions. The release of NO and the formation of nitrates from alkaline solutions with NO.sub.2 * or N.sub.2 O.sub.3 have been explained by water vapor or mist produced by the heat of neutralization.sup.(39,42,46). In addition, reaction 1-15 is reported.sup.(48) to occur in strong alkaline solutions to some extent depending upon the mass transfer conditions (liquid-phase transfer of HNO.sub.2 and liquid- and gas-phase transfer of NO). Also, two different mechanisms have been proposed for the reaction of NO.sub.2 and/or N.sub.2 O.sub.3 with sodium hydroxide in solution. One group.sup.(36,39,41,44) suggests that there are direct reactions (reactions 1-17 through 1-19); and the other group.sup.(42), suggests that the initial reactions occur with water (see reactions 1-3, 1-8, 1-12, 1-13, and 1-14).
______________________________________ Liquid Phase Reactions with NaOH.sub.(l) N.sub.2 O.sub.4(l) + 2NaOH.sub.(l) NaNO.sub.3(l) + NaNO.sub.2(l) + H.sub.2 O.sub.(l) (1-17) 2NO.sub.2(l) + 2NaOH.sub.(l) NaNO.sub.3(l) + NaNO.sub.2(l) + H.sub.2 O.sub.(l) (1-18) N.sub.2 O.sub.3(l) + 2NaOH.sub.(l) 2NaNO.sub.2(l) + H.sub.2 O.sub.(l) (1-19) Electrochemical Half-Cell Oxidation Potentials N.sub.2 O.sub.4(l) + 4OH.sup.-1 2NO.sub.3.sup.-1 + 2H.sub.2 O + 2e.sup.-1 -0.85 V (1-20) NO.sub.3.sup.-1 + H.sub.2 O.sub.(l) + 2e.sup.-1 NO.sub.2.sup.-1 + 2OH.sup.-1 -0.01 V (1-21) ______________________________________
The half cell reactions, 1-20 and 1-21, can be combined to give the oxidation-reduction reaction 1-17, which has a potential of -0.86 V. The free energy, .DELTA.G, for this reaction is negative, which say's that the reaction would occur spontaneously in the indicated direction. Therefore, direct reaction of nitrogen tetroxide with the hydroxide ion should occur spontaneously. Thus, there are two reaction possibilities for nitrogen tetroxide to absorb in alkaline solutions, either directly with water followed by neutralization or directly with the hydroxide ion. However, reaction kinetics indicate that the absorption of nitrogen tetroxide controls the process and that the reaction is pseudo-first order with respect to nitrogen tetroxide.sup.(37-39,43,46,49).
Chambers and Sherwood.sup.(42) measured the absorption of NO.sub.2 in sodium hydroxide (2.7 to 34.1-wt percent) and nitric acid (5.7 to 69.8-wt percent) in a wetted-wall tower and a batch absorption vessel. The ratio of the effective film thickness for water vapor to that for nitrogen dioxide is essentially the same as the reciprocal ratio of the gas film absorption coefficients. With water at the maximum for each curve, results are similar to the results obtained by Peters and Holman.sup.(41) who measured the removal efficiencies for water, 24-wt percent sodium chloride, and 20-wt percent sodium hydroxide.
Other process have been applied to waste streams in attempts to convert the waste to elementally rich compositions which could be used as fertilizer. U.S. Pat. No. 4,119,538 describes the conversion of organic and inorganic residues from fermentation processes to fertilizer by combining the waste liquor residue with inorganic ash.
U.S. Pat. No. 4,514,366 describes the production of liquid fertilizer by using liquid wastewater from a phosphorous smelting furnace. The process involves both the use of phosphorous sludge made at phosphorous furnaces to produce suspension fertilizer and the recovery of phosphorous liquid waste water from the smelting furnace.
U.S. Pat. No. 5,275,639 similarly reacts phosphorous containing sludges with ammonia to produce fertilizer with both phosphorous and nitrogen contributions.
U.S. Pat. No. 5,362,319 describes a process and apparatus for the treatment of unstable solids, such as scrubber solids. Oxidizing agents are provided to convert at least potassium, calcium or magnesium bisulfites to their corresponding sulfate forms. The partial oxidized residue of the sulfate intermediate product may then be completely oxidized by exposure to electromagnetic energy.
U.S. Pat. No. 5,447,637 describes the use of toxic liquid waste streams (phossy water) produced from elemental phosphorous reagent processes. The elemental phosphorous remaining in this toxic residue may be present as a solution, colloidal suspension or macroscopic particles. The phossy water is combined with a neutralizer, and then ammonia, phosphoric acid and suspending clay are combined to form a fertilizer product.